This invention relates to a high frequency dielectric resonator and a method for making the same.
As filters for use in equipment operating with a microwave in the frequency range from several hundred MHz to GHz, a great attention is now paid to dielectric resonators because of small size and high performance.
The dielectric resonator to which the present invention pertains is typically a coaxial dielectric resonator. Such a resonator includes a center-bored cylindrical body having an electrode formed and extending over one end surface and the outer and inner circumferential surfaces. That is, there is formed an electrode having a coaxial dielectric line having an electrically short-circuit surface on one end and an open-circuit surface on the other end. The loss in this type of dielectric resonator is a loss in the dielectric constituting the resonator plus a Joule loss due to high frequency current passing the electrode conductor. It is represented by the following equation, provided that Qu is an unloaded quality factor: EQU 1/Qu=1/Qd+1/Qc
wherein Qd is a dielectric loss and Qc is a conductor loss. In the formula, Qd and Qc generally have values of about 20,000 and about 1,000, respectively. This indicates that the value of Qu largely depends on the value of Qc of the electrode. The conductor loss Qc is, in turn, represented by the equation: EQU 1/Qc=.delta.(1/a+1/b)/2ln(b/a)
wherein a and b are the inner and outer diameters of the dielectric resonator, respectively, and .delta. is the skin depth given by the following equation: ##EQU1## wherein .sigma. is the electric conductivity of the metal, .mu. is the permeability of the metal, and f is the frequency. This equation indicates that Q of the electrode increases with the electric conductivity of the metal of which the electrode is made. The skin depth .delta. varies with frequency f as well as with the conductivity .sigma. of the metal. In general, the electrode thickness is made thicker several folds than skin depth .delta. to minimize radiation loss.
In the prior art, silver is used to form an electrode on a dielectric body for use at high frequencies such as microwave. A silver coating is generally formed on the surface of a microwave dielectric by transfer coating the surface with silver paste solution using a sponge impregnated therewith and then baking at a temperature of 700.degree. to 900.degree. C. The silver paste solution is a suspension of powder silver dispersed along with glass frit in a solvent having an organic binder dissolved therein. This method suffers from many drawbacks. For example, more working steps are required for coating of the outer, inner and end surfaces with silver paste and the resulting silver coating varies in thickness. A cost problem also arises because expensive silver must be thickly applied in order to achieve a critical coating thickness.
Solid silver in itself has the greatest electric conductivity among metals as demonstrated by its specific resistance of 1.62.times.10.sup.-6 ohm-cm. The actual sintered silver coating obtained by applying silver paste solution followed by baking has a film resistivity higher than that of solid silver by a factor of 1.15 to 1.80, which value is higher than the specific resistance of copper of 1.72.times.10.sup.-6 ohm-cm.
The silver paste solution also contains glass frit to achieve bonding with dielectric. The presence of glass frit at the interface between the silver film resulting from baking and the dielectric undesirably detracts from the electric conductivity of the skin depth layer which is most closely related to the loss. An additional problem of silver coarse-graining will occur when solder is applied to part of the electrode to complete the circuit.
In place of the above-mentioned method using expensive silver, another method using copper paste was proposed. The copper paste used is prepared by incorporating glass frit, organic binder, and solvent into copper powder and milling the mixture. Although some merits are obtained in material cost, not only the drawbacks associated with the silver paste method remain unsolved in this copper paste method, but the value of Q is further reduced.
It was also proposed to form a copper coating on a ceramic body by electroless plating as disclosed in Japanese Patent Application Kokai No. 54-108544.
It should be noted that the coating must be 5 or 6 .mu.m or thicker when the skin depth is taken into account. If copper is applied by electroless plating, it is difficult to form a dense and uniform copper coating. The resulting copper coating has a rough surface and a locally varying thickness. Further, many plating blisters frequently occur particularly on the inner circumferential surface of the dielectric resonator. Dielectric resonators having an electrode in the form of an electroless plated copper coating thus have many shortcomings including low electric conductivity .sigma., low and widely varying quality factor Q, and low bond strength.
A further attempt was made to eliminate these drawbacks by heat treating the electroless plated copper coating in an inert atmosphere of, for example, nitrogen and argon as disclosed in Japanese Patent Application Kokai No. SHO 58-166806. The heat treatment in an inert atmosphere improves the conductor loss Qc of the electrode and hence, the quality factor Q of the dielectric resonator to some extent. However, since no improvement is expected in film uniformity, denseness, and smoothness, there are achieved only slight improvements in electric conductivity, Q variation, and bond strength, failing to provide satisfactorily stable dielectric resonators.